Preheat the oven to 400ºF. Wash the mushrooms and cut them in half if they are large.

In a small dish, stir together the olive oil, balsamic vinegar, brown sugar, soy sauce, minced garlic, and thyme. Place the mushrooms in a casserole dish big enough to allow them to be in a single layer. Pour the balsamic marinade over top and stir until the mushrooms are coated.

Transfer the the mushrooms to the preheated oven and roast for 45 minutes, giving them a stir every 15 minutes. After 45 minutes the mushrooms should be tender and there should be a nice sauce in the bottom of the dish.